The overall objective of the proposed research is to characterize the purine base and nucleoside transport systems in rabbit reticulocytes, with a focus on determining whether, and to what extent, maturation of mammalian reticulocytes is associated with altered systems for transporting these nucleotide precursors. The research will be carried out in three stages. Initial kinetic characterization of transport of radioactively labelled purines into intact rabbit reticulocytes, obtained following phenylhydrazine treatment, will be followed by similar studies of reticulocyte ghosts and membrane vesicles--focusing on kinetic and regulatory parameters, as well as possible energy dependence and involvement of enzymes in the transport process. Comparison of the data obtained using reticulocytes with results of parallel studies of mature rabbit erythrocytes will indicate whether significant changes in transport mechanisms and/or regulation of transport carriers are correlated with known metabolic changes associated with reticulocyte maturation.